streaming radio / music options - can anyone summerize / point to a doc that explains options / costs?

Helping a friend that wants to get music from the web.  Can anyone comment on the options, prices, do they have ads / no ads, droid / iOS app, and a key point - do you need to acknowledge you are still there or does it just keep going?

I know I can get a couple FM stations I listen to - they keep playing for hours at a time.  Have ads, but free.  At least on the PC.  in the droid / ios apps, it may ask are you still there.

thanks!